“’71” is a gripping, crackerjack thriller about real-life horrors — no less than the Irish Troubles — but don’t let that throw you. It’s not a crassly exploitive time-killer — though its director, Yann Demange, could easily graduate to Hollywood — but a nail-biter with none of the black-or-white morality that tends to define the genre. Who is good bad becomes impossible to tell. It exists in the fog of war, where everyone at least dips a toe in the evil side, doing heinous things out of a base need to survive. Those who endure have to live with the wrongs that allowed them to carry on.

But it’s also a crackerjack thriller, and its pitilessness is matched only by its economy. “Unbroken”’s Jack O’Connell plays Gary, a young, fresh-faced British soldier and our token tabula rasa. He will be part of an underprepared unit dispatched to Belfast on a poorly considered peace-keeping mission, one that blooms into a full-on riot, whose messy violence leaves one of them shot in the head. In the madness, Gary is left behind, with a severe wound — a sitting duck who has to rely upon pluck, fast-thinking and strangers whose kindness may, out of necessity, eventually curdle.

This may look like a survive-the-night urban nightmare — a more politically engaged “Judgment Night,” whose handheld camerawork and fast cutting are both several coffees shy of mega-jittery Paul Greengrass — but Gary’s exit strategy is anything but clear. There’s no easy escape, and we soon learn that there’s considerable blurring between factions, including hotheaded military types (including a reliably nostril-flaring Sean Harris) who’ve worked out shady, rickety deals with their apparent enemies, who themselves may be less confident than they at first seem. Gary’s descent reveals a moral murk that doesn’t just involve one messy murder on his part; anyone who gets involved, even a noble father and daughter who happen upon him passed out in an alley, gets corrupted.

That “’71” is technically a thriller doesn’t cheapen its observations; it only strengthens its complexity. Characters who chase down our hero may seem like baddies in the heat of the moment, but they’ll become prey or even victims themselves later. “’71” doesn’t dig too deep into motivations or the politics of the Troubles; everyone’s a chess piece to be moved around. But that in itself underlines how people in such a strife wind up reduced to mere chess pieces themselves, dispensing with their humanity for causes they see as right, and which may be right. Quietly, and in the guise of trash cinema, it offers a clear-eyed view of an issue that cannot be contained.